The present invention relates to a virally safe, thrombin- and pyrogen-free factor-XIa concentrate and to the use of a virally safe factor-XIa concentrate for the production of a pharmaceutical composition for the treatment of coagulation disorders.
Injuries of the organism may result in severe bleeding, with massive or even fatal blood loss. The organism is protected against severe blood loss by a hemostatic mechanism, which causes the exiting blood to coagulate, whereby the bleeding is controlled and the wound is closed. Research into the hemostatic process has been conducted during the last century, and although a basic understanding has been gained, is still ongoing.
According to the present understanding, disruption of vessels exposes substances which trigger blood coagulation, so that a hemostatic plug can be formed. The plug contains a network of fibrin and platelets, the platelet count exceeding the blood count 20 to 200-fold. The plug adheres to, and closes, the injured site. The importance of the role of platelets in hemostasis can be illustrated by the fact that patients with afibrinogenemia do not tend to bleed profusely, while patients with severe thrombocytopenia may suffer non-controllable bleedings.
The blood coagulation process involves individual blood components such as blood cells, microparticles, and blood plasma. The coagulation of blood occurs when fibrinogen as a soluble protein in the blood plasma is converted into insoluble fibrin by the enzymatic action of thrombin, an enzyme which splits fibrinogen into soluble fibrin monomers and fibrinopeptides A and B. The fibrin monomers aggregate to fibrin monomer complexes and finally to insoluble fibrin. In patients with normal blood coagulation, only approximately one fourth of the fibrinogen that was present in the amount of blood from which a clot is formed, is converted into fibrin. The further conversion of fibrinogen in the clot depends on the amount of thrombin generated therein (Kumar R, et al.)
Thrombin results from prothrombin by activation at the end of the enzyme cascade, where coagulation factors which are pro-enzymes are activated into activated coagulation factors which are enzymes in a predetermined order. Coagulation factors which are not pro-enzymes are pro-co-factors, which are converted enzymatically into co-factors. Each co-factor enhances the enzymatic conversion of a specific pro-enzyme into an enzyme (Mann K G, et al.).
The enzyme cascade which results in thrombin can be divided into four different pathways, the extrinsic, the intrinsic, and the common pathway, and in addition, the so-called contact-phase. The present understanding is that on an injured blood vessel, tissue factor, a cell-bound lipoprotein, initiates the extrinsic pathway by forming a complex with factor VIIa, and this complex activates factor X into factor Xa. Factor Xa forms another enzyme complex, called prothrombinase, which generates thrombin from prothrombin. This pathway, where factor Xa generates thrombin, is called the common pathway. The generation of factor Xa by the extrinsic pathway is soon interrupted by tissue factor pathway inhibitor. As a consequence, only small amounts of thrombin can be generated via the extrinsic and common pathways. The small amounts of thrombin, however, trigger the activation of the intrinsic pathway. By activation of the intrinsic pathway large amounts of factor X are activated, so that thrombin is generated in excess via the common pathway. To what extent platelets and the contact phase contribute to the activation of the intrinsic pathway, is still a matter of discussion (Walsh P).
In patients who suffer from severe coagulation disorders all their lives, even minor traumatic events may cause uncontrollable bleedings. Such patients have either inherited deficiencies of specific coagulation factors or acquired the deficiencies in the course of their lives. Bleeding disorders of this type are referred to as hemophilia. Most patients who suffer from severe hemophilia have a deficiency of coagulation factor VIII (Brown S).
Coagulation factor VIII is a pro-cofactor, which is enzymatically transformed into a cofactor via the intrinsic pathway. This cofactor accelerates the activation of factor X by tenase, an activated factor-VIII-IX complex, dramatically, so that an excess of thrombin can be formed via the common pathway. Thrombin converts fibrinogen and factor XIII into fibrin and factor XIIIa, respectively. Factor XIIIa, a transglutaminase, causes the formed fibrin to cross-link, which results in an increased adhesion of the coagulated blood to the wound edges. In the further course, TAFI, a proenzyme, is activated into TAFIa. TAFIa splits off the receptor peptide for plasmin from fibrin, rendering the cross-linked fibrin more resistant against lysis. These enzymatic processes lead to an increased stability of the clot, an increase in its elasticity, and an increase in its resistance to lysis.
Patients with factor VIII deficiency form hemostatic plugs with difficulty only. If they do form a plug, the plug is fragile and will dissolve within short by fibrinolytic processes, the reason being that the intrinsic pathway is impaired in the clot and only an insufficient amount of thrombin is generated in the clot (Sixma J, et al.).
Once it became possible to produce concentrates of coagulation factor VIII as part of the fractionation of human plasma, bleeders with factor VIII deficiency could be treated so successfully that their average lifespan of 15 years could be prolonged to that of a normal person.
It was gradually understood that a considerable percentage of patients that had been treated successfully with factor VIII concentrates turned refractory to the very products they were treated with. Investigations of the phenomenon led to the conclusion that those patients developed antibodies against the homologous factor VIII they had received, which largely inhibited the function of factor VIII in the coagulation process. Such bleeders, thereafter referred to as factor VIII inhibitor patients, suffered the same fate as factor VIII deficient patients prior to the availability of factor VIII concentrates (van den Berg H, et al.). The same pathological process occurs in factor IX deficient patients as they become refractory to substitution therapy with factor IX concentrates.
In the 1970's, factor VIII inhibitor patients were treated unsuccessfully with different hemostatic medicinal products. It became known that only certain batches of products containing prothrombin complex were effective. Medicinal products containing prothrombin complex contain several vitamin-K-dependent coagulation factors, predominantly factors II, IX, X, and VII, and, depending upon the manufacturing process, those factors may in part be present in activated form (Kelly P, et al. and Kurczynski E, et al.). The Baxter Group then succeeded in manufacturing such partly activated prothrombin complex preparations and in marketing them by the trade name of “Autoplex” (Fekete L, et al.).
About the same time, Immuno AG succeeded in developing a product for the treatment of the same group of patients, which bridges the impairment in the coagulation cascade and normalizes coagulation (Eibl J, et al. and Turecek P, et al.). This product was introduced by the name of “FEIBA”. After the Baxter Group acquired Immuno AG in 1998, Baxter discontinued the production of Autoplex and continued producing and distributing FEIBA worldwide.
In the early 1980's, Novo Nordisk A/S introduced a recombinant factor VIIa product (Hedner U, et al.) by the name of “NovoSeven” (Hedner U) for the treatment of patients with factor VIII and factor IX inhibitors and other bleeding disorders. (Roberts H, et al.) Thus, two companies became leaders in parenteral medicinal products for the treatment of bleeders with inhibitors. As far as estimates allow, about 90% of the market today is covered by FEIBA and NovoSeven, amounting to annual sales of 1.5 billion Dollars, with NovoSeven accounting for approximately 60% and FEIBA approximately 40%.
The most recent publication about the efficacy of FEIBA and NovoSeven as assessed in a comparative multicenter trial reports no significant difference in the percentage of patients who stopped bleeding within six hours of treatment. Neither has their been a difference in the rate of adverse events, particularly thromboses (Astermark J, et al.).
Jan Astermark et al have shown that treatment with those products produced rapid hemostasis in only half of the patients. In about 25 percent of the patients, hemostasis occurred only after repeated doses of either Feiba or NovoSeven, and only after a prolonged period of time. Another 25 percent had either insufficient hemostasis, or the bleeding could not be controlled at all. The frequency of adverse events, particularly thromboses, was the same in either group.
Since the introduction and successful use of these pharmaceutical preparations, there has been speculation about their mode of action. In parallel, and irrespective thereof, the understanding of the coagulation process itself has changed and improved over the last 30 years. Nevertheless, no uniform or ultimate opinion has been reached about their mode of action (Roberts H, et al.).